1. Lightning
“Thunder is good, thunder is impressive; but it is lightning that does the work.” Mark Twain--

2. Electricity and Charge
There exist two types of charged particles:
Electron – negative charge e-
Proton – positive charge p+
An atom with an unequal number of each is called an ion
e-’s are mobile and move from atom to atom
Current is defined as moving in the direction that the positive charge “moves”
Note: the current is opposite the direction of electron flow!

3. How do clouds gain a charge?
Collision between a large graupel particle (traveling downward) and a small ice particle (traveling upward) leaves a net negative charge on the graupel and a net positive charge on the lighter ice particle. +
Graupel
Updraft
- +
Ice crystal

4. Illustrates three primary types of lightning: IC, CC, CG
Image from NASA
The lower part of a thundercloud is usually negatively charged. The upper area is usually positively charged. Lightning from the negatively charged area of the cloud generally carries a negative charge to Earth and is called a negative flash. A discharge from a positively-charged area to Earth produces a positive flash.
Illustrates three primary types of lightning: IC, CC, CG

5. Primary types of lightning
Cloud-to-Ground:
Most damaging and dangerous
Best understood
Most flashes originate near the lower negative charge center and deliver negative charge to Earth—a negative lightning strike
However, an appreciable minority of flashes carry positive charge to Earth—a positive lightning strike
Often occur during the dissipating stage of a thunderstorm's life
More common as a percentage of total ground strikes during the winter months.

6. Primary types of lightning
Intra-Cloud: (in-cloud)
Most common form
Occurs between oppositely charged centers within the same cloud
Usually takes place within the cloud
From the outside, it looks like a diffuse brightening that flickers
The flash may exit the boundary of the cloud such that a bright channel, similar to a cloud-to-ground flash, can be visible for many miles

7. Primary types of lightning
Inter-Cloud: (cloud-cloud)
Occurs between charge centers in two different clouds
Discharge bridges a gap of clear air between them
Additional types of electric phenomena above the cloud
Red sprites, blue jets, elves (flashes above the cloud toward the ionosphere)

8. Other Lightning Terms
Heat lightning – lightning is visible, but thunder is inaudible because the storm is too far away
Bead lightning – flash looks like a strand of beads
Sheet lightning – widespread glow of an intra-cloud flash
St Elmo’s fire – point discharges that occur when the environmental electric field is high, typically at the tips of sharp conductors that enhance the electric field
Often seen on the tall masts of sailing ships

9. Illustrates three primary types of lightning: IC, CC, CG
Image from NASA
The lower part of a thundercloud is usually negatively charged. The upper area is usually positively charged. Lightning from the negatively charged area of the cloud generally carries a negative charge to Earth and is called a negative flash. A discharge from a positively-charged area to Earth produces a positive flash.
Illustrates three primary types of lightning: IC, CC, CG

10. Transient Luminous Events
From NSSL:
Distance, km
From NSSL:

11. First color photograph of a sprite:
Taken from:

12. Steps to a Lightning Discharge
1) Stepped Leader – electrons move toward ground, met by ‘streamer’ coming up from the ground
2) Return Stroke – electrons flow to ground; therefore current travels upward; this is the flash
3) Dart Leader – moves through ionized channel from previous return stroke
4) Multiple return strokes until cloud discharges
The multiple strokes of lightning makes it appear to ‘flicker’

13. Thunder Lightning is very hot (~30,000ºC).
Violently expanding air causes an audible shock wave
Close Lightning
Thunder sounds like a “crack” or a loud explosion
Distant Lightning
Thunder sounds like a low rumble--we’re hearing the sound from different parts of the lightning channel
Why? Higher frequency sounds bend upward more easily (are refracted); lower frequencies travel farther near ground

14. Thunder How far away is that lightning? Lightning with no thunder?
Light travels much faster than sound
Count the time between when you saw the flash and heard the thunder, and divide by:
5 seconds per mile
3 seconds per km
Lightning with no thunder?
Could be too far away
Sound could be bent (refracted) by the atmosphere away from you

15. Thunder The person hears thunder from the bottom of the lightning
channel in 6 seconds. She will continue to hear the
thunder until sound from the
top of the channel arrives (2.8 km away) in 8.4 seconds.
2.8 km
2 km
2 km

16. Thunder The person hears thunder from the bottom of the lightning
channel in 8.4 seconds. She
will continue to hear the
thunder until sound from the
top of the channel arrives
(4.8 km away) in 14.4 seconds.
2 km
4.8 km
Long-lasting thunder suggests
extensive horizontal channel
2.8 km

17. Courtesy: NASA

18. Some Facts About Lightning
Lightning is an electrical discharge
Lighting typically extends ~5 km high and is 2 to 3 cm in diameter
Note: it appears thicker than it really is
Temperature in lightning channel can reach up to ~50,000°C (most ~30,000°C)
Five times hotter than the surface of the sun!
Kills an average of ~70 people per year in U.S.
Odds of you getting struck in your lifetime (assuming you live to be 80 years old) ≈ 1/5000

19. Average Number of Thunderstorm Days per Year
© 1998 Prentice Hall -- From: Lutgens and Tarbuck, The Atmosphere, 7th Ed.
Based on NLDN data

20. Average Number of Lightning Flashes per Year per km2
From:

21. Global lightning density map
Likely the average number of thunderstorm days per year; highest is ~~280 days/year in Uganda (??)
From:

22. Lightning Protection
Lightning rods act as locations where streamers can be launched upward toward descending stepped leaders
It’s better for the lightning rods to be struck, where the electrical energy can be safely transferred to ground, than the house
The “cone of safety” is ~45o under the lightning rod.

25. Lightning Protection
“Cone of Protection”
Fulgurite

26. Courtesy: NSSL

27. Courtesy: NSSL

28. Lightning striking away from storm:
Courtesy: NSSL

29. Lightning Striking a Lake
Taken from bigfoto.com

30. Great websites to check out:
Sprites and Jets:
Strike victims: